1. Field of the Invention
The invention relates to a high pressure mercury lamp and an emission device for a high pressure mercury lamp. The invention relates especially to a super high pressure mercury lamp in which a discharge vessel is filled with mercury in an amount at least equal to 0.15 mg/mm.sup.3, in which furthermore the mercury vapor pressure during operation is at least equal to a hundred and some dozen atm, and which is used as a backlight of a liquid crystal display device of the projection type or the like.
2. Description of the Related Art
In a liquid crystal display device of the projection type, there is a demand for operation of its light source in a horizontal operating position in order to keep the height of the device low. Furthermore, there is a demand for illumination of images onto a rectangular screen in a uniform manner and with adequate color reproduction. Therefore, a metal halide lamp of the horizontal operating type is used as the light source and is filled with mercury and a metal halide. Furthermore, recently, smaller and smaller metal halide lamps, and more and more often point light sources have been produced, and lamps with extremely small dimensions between the electrodes have been used in practice.
Against this background, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with a pressure at least equal to 200 bar (roughly 197 atm), have recently been proposed. Here, the increased mercury vapor pressure suppresses broadening of the arc (the arc is contracted) and a considerable increase of the light intensity is desired; this is disclosed, for example, in Japanese patent disclosure document HEI 2-148561 (U.S. Pat. No. 5,109,181) and in Japanese patent disclosure document HEI 6-52830 (U.S. Pat. No. 5,497,049).
In Japanese patent disclosure document HEI 2-148561 (U.S. Pat. No. 5,109,181), a high pressure mercury lamp is disclosed in which a discharge vessel which has a pair of tungsten electrodes is filled with a rare gas, at least 0.2 mg/mm.sup.3 mercury and a halogen in the range from 1.times.10.sup.-6 to 1.times.10.sup.-4 .mu.mol/mm.sup.3, and which is operated with a wall load at least equal to 1 W/mm.sup.2.
The following can be taken from this publication:
The reason for the amount of mercury added being greater than or equal to 0.2 mg/mm.sup.3 is to raise the mercury pressure, to increase the number of continuous spectra in the visible radiation range, especially in the red range, and to improve the color reproduction. The reason for the tube wall load of greater than or equal to 1 W/mm.sup.2 is to increase the temperature in the coolest portion in order to increase the mercury pressure. The reason for adding a halogen is to prevent blackening of the tube wall.
On the other hand, Japanese patent disclosure document HEI 6-52830 (U.S. Pat. No. 5,497,049) discloses that, in addition to the above described amount of mercury, the value ofthe tube wall load, and the amount of halogen, the shape of the discharge vessel and the distance between the electrodes is fixed and furthermore bromine is used as the halogen.
Here, the following is shown:
The reason for adding the bromine is to prevent blackening of the tube wall. When at least 10.sup.-6 .mu.mol/mm.sup.3 bromine is added, an adequate effect is achieved. At amounts greater than 10.sup.-4 .mu.mol/mm.sup.3, etching of the electrodes occurs.
On the other hand, one such super high pressure mercury lamp is operated horizontally, i.e., it is operated in such a way that the virtual line which forms between the electrodes is parallel to the horizontal. In this case, as a result of the floating of the arc which forms between the electrodes in the upper region of the discharge vessel, the thermal load is extremely high, while in the lower area of the discharge vessel, the thermal load becomes low. To obtain a high operating pressure of the mercury, it is necessary to make the discharge vessel smaller, i.e., to reduce the inside diameter of the emission space. However, if the latter becomes too small, the fused silica glass comprising the discharge vessel crystallizes. The range of reduction of the discharge vessel is therefore limited.
On the other hand, recently, a liquid crystal projection television has attracted attention; in it, in the main part of the television, there is a discharge lamp as the light source for purposes of illumination from behind the television picture, i.e., a so-called rear projection type television. In this television, the discharge vessel need not necessarily be operated horizontally with respect to optical construction, but it can also be operated vertically.
In vertical operation of the above described super high pressure mercury lamp, the effect of the thermal load within the discharge vessel is completely different than in the case of horizontal operation. Specifically, areas with high and low thermal load do not form on the tube wall in the vicinity of the arc, but at the base points of the electrodes. Also the influence of gas convection in the discharge vessel changes greatly. The above described publications of the prior art do not describe whether the discharge vessel is being operated horizontally or vertically. Here therefore, in this respect, the influence of thermal load and gas convection within the discharge vessel is ignored.
In a television of the rear projection type, there is a demand for high picture quality. Especially in the case of using the above described super high pressure mercury lamp as the light source, is it regarded as a disadvantage that the illuminance of the picture fluctuates due to the radiance spot of the cathode with high radiance flickering due to gas convection or the like on the faces of the electrodes.